SCIENCE

Development of light sources

James Hooker, Fibre Optic Lamp Co Ltd, on 26 October 2001

Sir Humphrey Davy produced electric light for the first time in 1802 using a carbon arc. Two pointed carbon rods were connected to the positive and negative terminals of the supply, touched together and then slightly separated so that a continuous spark, an arc, formed between them. This heated the end of one rod to over 2000 deg.C so that a bright, white light was produced. The problem was that the carbon burnt away and the space between the rods had to be continuously adjusted to maintain the arc. The light was also very bright and only really suitable for use outdoors. The system is still in use for searchlights and projectors.

It was not until 1897 that an alternative was developed. Joseph Swann in England and Edison in USA both produced Incandescent Filament lamps. Swann did not publish or patent his work immediately but Edison did so, and got the credit for the invention. Edison produced a carbon filament from bamboo; Swann started from cellulose. Both were successful designs but were expensive because the filaments had to be enclosed in an evacuated glass bulb to prevent them burning away; this required the use of platinum wires to lead the current through the glass, and platinum is very expensive.

The replacement for carbon for the filament was developed by a team under William Coolidge in 1907. Tungsten is a metal with a very high melting point that can be formed into wire. In its basic form this wire is very brittle but by swaging it a ductile wire can be produced. The final wire is thinner than a human hair and, in a 100w bulb, there is over a metre length. At first this was arranged as a `squirrel cage' which required a large bulb. Later the wire was coiled and today it is double-coiled. The conductors through the glass envelope are now made of a nickel-iron alloy encased in copper.

The modern tungsten filament light bulb is made in 300 operations using 80 different materials for a production cost of 8p. The machines which make the glass envelopes from a special glass with high electrical resistance at high temperature have a higher production rate than any in the world 120,000 bulbs per hour.

To prevent the blackening of the glass by the evaporated tungsten, and to make the bulbs smaller, halogen gas at 10 atmospheres pressure was introduced into the bulb. A demonstration illustrated how this kept the glass clear by reacting with the tungsten vapour (to form tungsten bromide) and re-depositing the metal on the cooler parts of the filament, thus releasing the bromine again. When the bromine was condensed with liquid nitrogen the bulb darkened quickly; it cleared when the bromine was re-vapourised.

Incandescent filament lamps supply visible light from only 10% of the energy fed to them; 30% is in infra-red wavelengths. Work is now in hand to recover some of this I.R. energy by coating the bulb. Other long-term work is studying ceramic filaments.

But the current alternative is a Discharge Lamp a fluorescent tube. This produces a discharge through an inert gas or metal-vapour inside a tube coated with fluorescent material which produces visible light from the ultra-violet of the discharge. These are much more efficient in the use of the energy; the sodium vapour street lamp with its yellow light is the most efficient producing 200 lumens/watt compared to about 15 for an incandescent filament lamp. The phosphor powder used to coat the inside of the tube of a fluorescent lamp is a mixture of calcium tungstate (blue light), zinc silicate (green), and yttrium oxide (red) in various proportions to give `warm white' and other variants of `white' light. Mercury vapour lamps give the nearest rendering to true white.

The Cold Cathode Fluorescent Lamp is the latest development of this type. It uses smaller diameter tubes bent to form a `bulb' for domestic use, which has a much longer life than the 1000 hours of the standard Incandescent Filament lamp.

The next development, which may reach the shops in a few years time, is the Electrodeless Induction Lamp, which has no filament but works at radio frequency. It could have a life of 80,000 hours and an output of 80 lumens/watt. At the moment the cost of the radio frequency control unit is very high but some of these lamps are available at about £500 each.

The last type Mr Hooker described and demonstrated was the L.E.D. Light Emitting Diode which is widely used in modern cars in the back window as brake lights. These are very efficient and long-lasting, small in size, but only produce coloured light; red, green and blue are available. White light can be produced by mixing the light from these three colours

Questions were fired at the speaker as he spoke and he dealt with each of them thoroughly.

Donald Lovell